The present invention relates to a method and apparatus for use in the clinical monitoring of an indirect blood pressure measurement of a body.
In a well known indirect blood pressure measurement system utilizing an inflatable cuff, the cuff pressure is increased until the artery is completely occluded, and while the cuff pressure is gradually decreased, a blood flow is started overcoming a certain cuff pressure and a Korotkov sound synchronized with a heart pulse appears. The cuff pressure value existing when the first Korotkov sound appears is defined as a systolic blood pressure. When the cuff pressure is further decreased to another certain cuff pressure, the Korotkov sound disappears and this latter cuff pressure value is defined as a diastolic blood pressure.
Most of the presently utilized automatic blood pressure measuring systems detect the appearance and disappearance of Korotkov sounds in order to determine the systolic and diastolic blood pressure levels as described above.
Since, the volume and quality of Korotkov sounds depends in large measure on the characteristic of each body, it is very difficult to precisely detect the appearance or disappearance of the Korotkov sounds from different individuals as signals separate from noise which may, for example, be caused by, a person touching the cuff.
Several prior art techniques have been developed which attempt to overcome this difficulty and precisely detect the appearance or disappearance of Korotkov sounds. In one technique, for example, when a Korotkov sound is found which is synchronized with a heart pulse, it is regarded as a true Korotkov sound and every other sound is deemed to be noise. A technique for determining a diastolic blood pressure is to note the cuff pressure existing when the last Korotkov sound appears if, after the above systolic blood pressure determination, no following Korotkov sounds appear after disappearance of a Korotkov sound for a few seconds, or if only heartbeat pulses appear.
None of the above techniques is adequate to determine a blood pressure, especially a diastolic blood pressure, where the time of disappearance of the Korotkov sound is not very clear. For example, in the technique where a disappearance is determined when no Korotkov sound is detected for a predetermined period of time, noise which appears within the predetermined period of time is often regarded as a Korotkov sound which begins a new predetermined period of time from the appearance of the noise which must transpire before the diastolic measurement is made. In other words, the noise appearing around the time of disappearance of a Korotkov sound causes the determination of a disastolic blood pressure to be erroneous, even if the noise is relatively minor. Similar problems exist where the disappearance of Korotkov sound is determined when a predetermined number of heart pulses occur unaccompanied by Korotkov sounds.
The present invention, therefore, provides a determination system for a true Korotkov sound wherein a logic "1" is established when a Korotkov sound synchronized with a heart pulse is detected, and a logic "0" is established when only a heart pulse is detected unaccompanied by a Korotkov sound. The logic signals are successively stored in a four bit memory, and every Korotkov sound is determined as a true Korotkov sound when three or four "1" signals exist in the four consecutive bit locations in the memory. A systolic blood pressure determination is made following the true Korotkov sound determination from the cuff pressure existing when the first Korotkov sound among the three or four detected Korotkov sounds is detected. The present invention further provides a counter in addition to the above memory which counts a "0" signal applied to the memory and is reset by the true Korotkov sound determination. A determination for a diastolic blood pressure is effected when the counter counts up three "0" signals, following which the cuff pressure when the last "1" signal appears as a true Korotkov sound is regarded as a diastolic blood pressure. In other words, when a "1" signal synchronized with a heart pulse appears in the memory, it is not always regarded as a true Korotkov sound for the determination of a blood pressure. However, when two or more "1" signals are stored in the above memory prior to the above "1" signal, each "1" signal in the memory is regarded as a true Korotkov sound. Since each "1" signal except those meeting the above criteria is regarded as noise, the measurement of blood pressure is not disturbed. This is especially useful in the diastolic blood pressure determination.
It is therefore a general object of the present invention to provide a method and apparatus for accurately measuring a systolic and diastolic blood pressure.
It is specific object of the present invention to provide a method and apparatus for determining a true Korotkov sound for use in measuring blood pressure.
It is another object of the present invention to provide a method and apparatus for determining a diastolic blood pressure by counting a predetermined number of heart pulses unaccompanied by a Korotkov sound after a systolic blood pressure determination and the disappearance of true Korotkov sounds as the cuff pressure is decreased during measurement.
It is a further object of the present invention to provide a method and apparatus for measuring a blood pressure of the body when an auscultatory gap exists.
Further and more specific objects and advantages of the present invention will be apparent from the following more particular description of a preferred embodiment of the invention, which is illustrated in the accompanying drawings.